U.S. patent number 6,415,864 [Application Number 09/726,294] was granted by the patent office on 2002-07-09 for system and method for separately producing water and oil from a reservoir.
This patent grant is currently assigned to Schlumberger Technology Corporation. Invention is credited to Min-Yi Chen, Brindesh Dhruva, Peter A. Goode, Rod F. Nelson, Terizhandur S. Ramakrishnan, Raj Kumar Michael Thambynayagam.
United States Patent |
6,415,864 |
Ramakrishnan , et
al. |
July 9, 2002 |
System and method for separately producing water and oil from a
reservoir
Abstract
A system for reservoir control. The system allows segregated
production of fluids, e.g. water and oil, to control the
fluid-fluid interface. Downhole sensors are utilized in providing
data about the location of the interface. This permits the
proactive monitoring and control of the interface prior to unwanted
intermingling of fluids, e.g. oil and water, during production.
Inventors: |
Ramakrishnan; Terizhandur S.
(Bethel, CT), Dhruva; Brindesh (Danbury, CT),
Thambynayagam; Raj Kumar Michael (Ridgefield, CT), Chen;
Min-Yi (West Redding, CT), Goode; Peter A. (Houston,
TX), Nelson; Rod F. (Sugar Land, TX) |
Assignee: |
Schlumberger Technology
Corporation (Sugar Land, TX)
|
Family
ID: |
24918004 |
Appl.
No.: |
09/726,294 |
Filed: |
November 30, 2000 |
Current U.S.
Class: |
166/250.03;
166/250.15; 166/54.1; 166/66; 166/369; 166/250.17; 166/306 |
Current CPC
Class: |
E21B
43/32 (20130101); E21B 47/047 (20200501) |
Current International
Class: |
E21B
43/00 (20060101); E21B 43/32 (20060101); E21B
47/04 (20060101); E21B 047/04 () |
Field of
Search: |
;166/250.01,254.1,250.03,250.15,250.17,305.1,306,369,53,54.1,65.1,66 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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WO 98/36155 |
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Aug 1998 |
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WO |
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WO 99/15755 |
|
Apr 1999 |
|
WO |
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WO 00/14381 |
|
Mar 2000 |
|
WO |
|
Other References
Gunning, J., Paterson, L., Poliak, B. Coning in dual completed
systems, J. Pet. Sci. Engng., 23:27-39, 1999. .
Muskat, M. Physical principles of oil production. McGraw-Hill, New
York, 1949. .
Ramakrishnan, T.S. and Wilkinson, D. Water-cut and fractional-flow
logs from array induction measurements. SPE Reservior Eval.,
2(1):85-94, 1999. .
Swisher, M.D. and Wojtanowicz, A.K. In situ-segregated production
of oil and water--A production method with environmental merit:
Field application, Soc. Pet. Eng., 43-50, 1995. .
Waxman, M.H. and Smits, L.J.M. Electrical conductivities in
oil-bearing shaly sands. Soc. Pet. Eng. J., 8:107-122, 1968. .
Wojtanowicz, Andrew K.; Downhole Water Sink (DWS) Technology
Initiative; 14 pages. .
Shirman, Ephim I.; A Well Completion Design Model for Water-Free
Production from Reservoirs Overlaying Aquifers; SPE International
Student Paper Contest; Oct. 6-9, 1996; 8 pages; Society of
Petroleum Engineers; Richardson, TX, U.S.A. .
Swisher, M.D. & Wojtanowicz, A.K.; New Dual Completion Method
Eliminates Bottom Water Coning; SPE 30697; Oct. 22-25, 1995; 7
pages; Society of Petroleum Engineers; Richardson, TX, U.S.A. .
Wojtanowicz, Andrew K., Hui Xu, & Bassiouni, Zaki; Segregated
Production Method for Oil Wells With Active Water Coning; Journal
of Petroleum Science and Engineering; 1994; 15 pages; vol. 11.
.
Meritorious Engineering Award Winners; Petroleum Engineer
International; Jun. 1996; 2 pages; Houston, TX, U.S.A..
|
Primary Examiner: Schoeppel; Roger
Attorney, Agent or Firm: Fletcher, Yoder & Van
Someren
Claims
What is claimed is:
1. A method for reducing watercut during the production of a
desired production fluid from a well having a wellbore lined by a
wellbore casing, comprising:
perforating the wellbore casing proximate a production fluid zone
and a water zone to permit ingress of a production fluid and water
into the wellbore;
producing the production fluid from the production fluid zone;
removing the water from the water zone to reduce watercut into the
production fluid zone;
sensing the location of an interface between the production fluid
and the water via a sensor array deployed external to the wellbore
casing; and
adjusting the rate at which at least one of the production fluid
and the water moves into the wellbore based at least in part on the
location of the interface.
2. The method as recited in claim 1, wherein producing comprises
producing a petroleum product.
3. The method as recited in claim 2, wherein sensing comprises
utilizing an electrode sensor array.
4. The method as recited in claim 3, further comprising obtaining a
plurality of output values from the electrode sensor array and
utilizing those values in a reservoir model to determine whether a
change in the flow rate of the petroleum product or the water is
desired.
5. The method as recited in claim 3, wherein adjusting is based
directly on a plurality of output values from the electrode sensor
array.
6. The method as recited in claim 3, wherein producing the
petroleum product comprises producing the petroleum product through
a completion.
7. The method as recited in claim 6, wherein removing the water
comprises removing the water via a second completion.
8. The method as recited in claim 7, wherein the completion
comprises an electric submersible pumping system.
9. The method as recited in claim 8, wherein the second completion
comprises a second electric submersible pumping system.
10. The method as recited in claim 7, wherein removing comprises
removing the water to a location at the surface of the earth.
11. The method as recited in claim 7, wherein removing comprises
reinjecting the water at a subterranean location.
12. The method as recited in claim 7, wherein the completion
comprises a control valve.
13. The method as recited in claim 12, wherein the second
completion comprises a second control valve.
14. The method as recited in claim 3, wherein utilizing includes
deploying at least one electrode of the electrode sensor array as a
current emitter.
15. A method for determining and controlling the location of a
fluid-fluid interface along a wellbore used in the production of
oil, comprising:
deploying a plurality of sensors along an exterior of the wellbore
above and below the fluid-fluid interface; and
outputting a signal from each sensor to indicate the presence of a
first fluid or a second fluid.
16. The method as recited in claim 15, wherein outputting comprises
outputting a signal indicative of oil as the first fluid.
17. The method as recited in claim 16, wherein outputting comprises
outputting a signal indicative of water as the second fluid.
18. The method as recited in claim 17, further comprising:
adjusting at least one of an oil production rate and a water
production rate based on the signals output from the plurality of
sensors.
19. The method as recited in claim 18, wherein deploying comprises
deploying an electrode array having a plurality of electrodes able
to output a voltage signal indicative of the presence of oil or
water.
20. The method as recited in claim 19, wherein deploying comprises
deploying at least one electrode that is a current emitter.
21. The method as recited in claim 19, wherein deploying comprises
locating the plurality of electrodes external to a wellbore casing
lining the wellbore.
22. The method as recited in claim 21, further comprising
determining the height of a hump in the oil-water interface remote
from the wellbore.
23. The method as recited in claim 19, wherein adjusting comprises
pumping the oil via an electric submersible pumping system.
24. The method as recited in claim 23, wherein adjusting comprises
pumping the water via a second electric submersible pumping
system.
25. The method as recited in claim 24, wherein pumping the water
includes directing the water to a subterranean injection
location.
26. The method as recited in claim 15, wherein outputting comprises
outputting a signal indicative of a gas as the first fluid.
27. A system for controlling an oil-water interface disposed about
a wellbore utilized in the production of an oil, comprising:
a first completion disposed within the wellbore for producing
oil;
a second completion disposed within the wellbore for producing
water; and
a sensor array disposed along the wellbore across an oil-water
interface formed between the oil and the water, wherein at least
one of the first and the second completions may be controlled to
adjust the location of the oil-water interface based on output from
the sensor array.
28. The system as recited in claim 27, wherein the sensor array
comprises a plurality of electrodes able to output signals that may
be used to determine the presence of an oil or a water.
29. The system as recited in claim 28, wherein the sensor array
comprises at least one electrode that is a current emitter.
30. The system as recited in claim 28, wherein the wellbore is
lined by a wellbore casing and the plurality of electrodes are
positioned outside the wellbore casing.
31. The system as recited in claim 28, wherein the first completion
comprises a control valve.
32. The system as recited in claim 28, wherein the first completion
comprises an electric submersible pumping system.
33. The system as recited in claim 28, wherein the second
completion comprises a control valve.
34. The system as recited in claim 28, wherein the second
completion comprises an electric submersible pumping system.
Description
FIELD OF THE INVENTION
The present invention relates generally to the production of oil
and water from a reservoir to limit the watercut or water coning
effects, and particularly to a system that utilizes an array of
sensors for sensing the oil and water interface to permit better
control over the movement of that interface.
BACKGROUND OF THE INVENTION
In some oil reservoirs, the oil production rate has been limited by
the inability to produce oil devoid of water. In vertical wells,
the upper limit of oil production rates has been limited by
watercutting, sometimes referred to as water coning, where water is
drawn into the oil zone perforations.
Water coning is caused by a hydraulic potential difference between
the fluid in the perforations and in the aquifer. Basically, the
radial pressure drop due to oil flow causes water to rise towards
the oil perforations. The rise of water to the oil perforations may
be limited by reducing the rate of oil production but this, of
course, greatly limits the "clean" oil production rate.
Attempts have been made to produce both oil and water from
appropriately located oil perforations and water perforations to
prevent the draw of water into the oil perforations. The water
perforations are formed through the wellbore casing, and water is
removed from the aquifer through the perforations at a rate that is
estimated to reduce water coning. One problem in existent systems
is the difficulty of controlling the production rates of oil and
water to ensure that neither water coning nor oil coning into the
water perforation occurs. Because there is no dependable way to
determine the advent of water coning or oil coning, the production
rates of oil and/or water are adjusted only when water is found in
the produced oil or oil in the produced water. Once this occurs,
however, the produced oil or water is no longer clean, and
sometimes the coning effect is difficult to reverse.
SUMMARY OF THE INVENTION
According to the present technique, a sensor array is utilized at a
downhole location across the oil-water interface. The sensors are
designed to output signals from which the presence of oil or water
may be determined. The outputs generated are used, for instance,
either directly or in a model based on reservoir characteristics.
The sensors permit detection of movement in the oil-water interface
which, in turn, allows the production rate of oil and/or water to
be changed in a manner that will compensate for the movement in the
oil-water interface. Thus, the effects of water coning or oil
coning can be detected and limited or reversed at an early stage of
development.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will hereafter be described with reference to the
accompanying drawings, wherein like reference numerals denote like
elements, and:
FIG. 1 is a front elevational view of an exemplary dual completion
used for the production of oil and water;
FIG. 2 is a front elevational view of an alternate dual completion
production system similar to FIG. 1;
FIG. 3 is another alternate embodiment of the dual completion
production system illustrated in FIG. 1;
FIG. 4 is an alternate embodiment of an oil and water production
system in which the water is reinjected at a separate subterranean
location;
FIG. 5 is a front elevational view of a system for producing
liquids from two separate production zones;
FIG. 6 is a flow chart illustrating an exemplary methodology for
utilizing data from sensors disposed through the interface between
the produced fluids;
FIG. 7 is an illustration similar to FIG. 5 showing additional
parameters of an interface formed between the produced liquids;
FIG. 8A is a graphical representation of changes in the sensor
output relative to changes in the oil-water interface; and
FIG. 8B is another graphical representation of changes in the
oil-water interface relative to changes in sensor output.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the following description, dual completions are used to produce
two liquids from a subterranean location. The differing types of
liquid are detected and the production rate of each liquid is
selected to control the interface formed between the liquids. In a
typical application, the system is utilized to enhance the
production of "clean" oil when an oil-water interface is formed
between oil at a high subterranean zone and water at a contiguous,
lower subterranean zone. Although the following discussion focuses
on the production of oil and the oil-water interface commonly found
in certain reservoirs, the system is not limited to use with those
specific liquids.
A variety of dual completions designs may be used for the
production of oil and water, as known to those of ordinary skill in
the art now and in the future. However, a few general applications
are described herein to enhance an understanding of the system and
method for controlling the production of oil and water in a way
that limits the formation of water coning or oil coning.
Referring generally to FIG. 1, a production system 10 for the
controlled production of oil and water is illustrated. System 10
includes a dual completion having a water production completion 12
and an oil production completion 14. The dual completion is
deployed in a wellbore 16 typically lined by a wellbore casing
18.
Wellbore casing 18 includes a plurality of openings through which
production fluids flow into wellbore 16. For example, the plurality
of openings may include a set of oil perforations 20 through which
oil flows into wellbore 16 for production along a fluid flow path
22. Similarly, wellbore casing 18 includes a plurality of water
perforations 24 through which water flows into wellbore 16.
In the illustrated embodiment, wellbore 16 is formed in a
geological formation 26 having an oil zone 28 disposed generally
above a water zone 30. Oil perforations 20 are located within oil
zone 28 to permit the inflow of oil into wellbore 16, and water
perforations 24 are disposed within water zone 30 to permit the
inflow of water into wellbore 16. An oil-water interface 32 forms
the boundary between the oil zone 28 and the water zone 30 and is
preferably maintained between oil perforations 20 and water
perforations 24. As described above, in the event removal of oil
from oil zone 28 is at too great a rate relative to the production
of water from water zone 30, water coning can occur in which water
cuts into the production of oil and enters oil perforations 20.
Contrariwise, if the relative production rate of water from water
zone 30 is too great, oil coning can occur where the oil-water
interface 32 is drawn towards water perforations 24 until oil is
drawn through perforations 24.
Within wellbore 16, the inflow of oil from oil zone 28 is separated
from the inflow of water through water perforations 24 by a
separation device, such as a packer 34. Packer 34 is deployed to
permit the separate production of water through water completion 12
and oil through oil completion 14. Effectively, packer 34 divides
the wellbore 16 into a lower water zone and an upper oil zone by
preventing mixing of oil and water after the liquids enter wellbore
16.
Changes in the position of the oil-water interface 32 are detected
by a plurality of sensors 36 disposed along wellbore 16 and across
oil-water interface 32. Individual sensors of the array of sensors
36 are designed to detect the presence of a given liquid. A signal
is output from each sensor to indicate the presence of, for
example, either oil or water. Thus, movement of the oil-water
interface 32 can be detected as it moves vertically from one sensor
to the next along wellbore 16.
One exemplary plurality of sensors 36 includes an array of
electrodes. The array of electrodes permits real-time sensing and
controlling of the production rates of oil and/or water. Due to the
conductivity contrast between oil and water, the electrodes provide
direct information regarding the movement of the oil-water
interface. For example, the electrodes can be used as passive
voltage measuring sensors able to output a signal indicative of the
presence of oil or water. In an exemplary embodiment, one or more
of the electrodes are used for current transmission while the
remaining electrodes are used as passive voltage measuring sensors.
In the embodiment illustrated, the electrodes extend along the
exterior of wellbore casing 18 above, between and below the oil
perforations and the water perforations.
The signals output by sensors 36 are transferred to a receiving
station 38 that preferably also functions as a controller for
controlling the flow rates of liquid through one or both of the
water completion 12 and the oil completion 14. Specific use of the
data received from sensor array 36 may vary depending on the
specific environment and application. For example, the data of
voltages/currents, pressures and flow rates may be used in
conjunction with a reservoir model. In this application, a
reservoir model is constructed to compute values for various
production and formation parameters, e.g. given the flow rates and
reservoir parameters, saturation levels, conductivities, pressures
and the electrode potentials may be computed. The computed values
are compared to the observed data, and the reservoir model is
iteratively updated. The fluid production rates are adjusted
according to new optimization calculations for the model.
In another exemplary application, the data output from sensors 36
is used directly rather than in conjunction with a reservoir model.
In this approach, an estimate for the desired electrode sensor
values or interface locations is made and a control algorithm is
determined to adjust the flow rate(s) of the oil and/or water in a
manner that maintains the electrode sensor values or the estimated
oil-water interface at a desired level. This approach allows direct
observation of the formation rather than carrying out reservoir
model updates. This latter approach may be called observation-based
control.
Regardless of whether the sensor data is used directly or in
conjunction with a reservoir model, the receiving
station/controller 38 utilizes the data output by sensors 36 to
adjust one or both of the flow rates of water and oil. For example,
in one type of production application, controller 38 is coupled to
an oil control valve 40 and a water control valve 42, shown
schematically in FIG. 1. Control valves 40 and 42 can be adjusted
to permit increased or decreased flow of oil and/or water.
Receiving/control station 38 can be constructed according to a
variety of designs. The station could be constructed to present
information from sensors 36 to an operator who would then, based on
this information, adjust the oil and/or water flow rates.
Alternatively, receiving station 38 can utilize a computer
programmed to appropriately analyze the data received from sensors
36 and automatically adjust one or both of the oil and water flow
rates, as would be understood by one of ordinary skill in the
art.
Also, a variety of sensors 36 can be used to detect the oil-water
interface 32. However, when sensors, such as electrodes are
utilized, the sensors preferably are deployed along wellbore 16
external to wellbore casing 18. This permits direct contact of
sensors 36 with the surrounding oil or water.
In FIG. 1, a representative system for producing oil and water is
illustrated, but a variety of systems may be utilized. For example,
rather than control valves, an electric submersible pumping (ESP)
system 44 may be utilized, as illustrated in FIG. 2. In this
embodiment, an ESP system pumps or produces water along a water
flow path 46. Receiving/control station 38 is utilized in selecting
the appropriate operating speed of electric submersible pumping
system 44 to control the flow of water based on the output from
sensors 36. Depending on the type of formation and the natural
pressure acting on oil zone 28, the production of oil along oil
flow path 22 may be controlled by, for example, a control valve or
another electric submersible pumping system. Electric submersible
pumping systems are used, for instance, when the natural well
pressure is not sufficient to raise the liquid to the surface of
the earth.
One example of a dual completion having dual electric submersible
pumping systems is illustrated in FIG. 3. In this embodiment, water
is produced along water flow path 46 by electric submersible
pumping system 44, and oil is produced along oil flow path 22 by a
second electric submersible pumping system 48. By way of example,
water may be produced through a tubing string 50, and oil may be
produced through a second tubing string 52. The pump speed of
either or both electric submersible pumping systems 44 and 48 may
be adjusted to control one or both of the oil and water production
rates and consequently the oil-water interface proximate wellbore
casing 18.
Alternatively, water may be produced to a subterranean location 54,
as illustrated in FIG. 4. In this exemplary embodiment, electric
submersible pumping system 44 directs the water downwardly along
water flow path 46 through a tubing 56. Tubing 56 extends through a
lower packer 58 that separates the water intake portion of wellbore
16 from the water injection portion of wellbore 16. As illustrated,
water is discharged beneath lower packer 58 into wellbore 16
through a discharge end 60. The water is then forced or injected
into formation 26 through a plurality of perforations 62. Again,
the production rates of oil and/or water can be controlled based on
data received from sensors 36, e.g. electrodes disposed along the
exterior of the wellbore casing above, between and below
perforations 20, 24 and 62. Thus, a variety of oil and water
production systems can be utilized in controlling oil-water
interface 32.
The data output from sensors 36 can be utilized in a variety of
ways to observe and control the oil-water interface 32.
Accordingly, the model based control and observation based control
methods discussed herein are merely exemplary utilizations of the
data provided. For purposes of this discussion, it may be assumed
that sensors 36 comprise an electrode array disposed on the outside
of wellbore casing 18.
In the model based control example, geological formation 26 is
initialized with available knowledge, such as seismics, the known
geology and wellbore logs. Properties, such as permeabilities,
capillary pressures and relative permeabilities are estimated,
often based on core data obtained for the specific geological
formation, as known to those of ordinary skill in the art.
Based on this available knowledge, a reservoir simulation program
(e.g. ECLIPSE.TM., available from Geoquest) is run to determine
optimal completion distances z.sub.o and z.sub.w, as illustrated in
FIG. 5. The distance z.sub.o represents the distance between oil
perforations 20 and the original oil-water interface 32. Similarly,
the distance z.sub.w represents the desired distance between water
perforations 24 and the oil-water interface 32. Based on formation
properties, the estimated flow rates of water (q.sub.w) and oil
(q.sub.o) out of formation 26 also are estimated. When water
completion 12 and oil completion 14 are operated to achieve the
estimated flow rates, a full flow simulation may be carried out
based on the flow rates and the formation properties. For example,
saturation and concentration data may be used to estimate
current/voltages at the various electrode sensor locations.
Typically, the saturation and concentration data is converted into
conductivity values through suitable petrophysical transformations
to facilitate comparison of the estimated current/voltages at the
sensor locations with the actual data provided by sensors 36. All
of the accumulated data at various time points may be compared with
the actual measured values from sensors 36 to update parameters of
the model and predict optimal production values on an iterative
basis, e.g. according to the least squares method.
The general reservoir model approach is illustrated best in FIG. 6.
As discussed above, flow and formation data 62 from a flow control
device 63, e.g. a pump or valve, are utilized in creating a model
of the reservoir 64. From this reservoir model, petrophysics (block
66) can be utilized to convert saturation and salinity distribution
data 68 into estimated conductivity distributions 70 across
electrode array 36. The conductivity distributions are applied to
an electromagnetics model 72, and compared with actual output from
electrodes 36. The actual electrode responses 73 are used with
other data 74, e.g. pressure and voltage data, to initialize and
update flow rates (see reference numeral 75) and, consequently, the
flow data 62 used by reservoir model 64. Typically, the electrode
responses 73, data 74, and computed data 77, e.g. computed
pressures, are compared and used to update flow rates on an
iterative basis (see block 76). Based on the continuously updated
reservoir model, the production rates of oil (q.sub.o) and/or water
(q.sub.w) are adjusted to maintain a desired oil-water interface at
a location that mitigates or reduces water coning. As recognized by
those of ordinary skill in the art, the actual reservoir model and
the data utilized in constructing and updating the model may vary
between reservoirs and applications.
Alternatively, an observation based control methodology may be used
to limit oil and water incursion into the water and oil
completions, respectively. Control of the oil and/or water
production can be accomplished based either directly on the sensor
voltages/currents or through the estimated interface location.
Production control, based directly on the sensor voltages/currents,
relies on the difference between measured sensor values and the
desired sensor values determined from knowledge of the sensor
physics and output relative to surrounding environment. If, on the
other hand, the control is based on estimated interface location, a
control algorithm is utilized to maintain the oil-water interface
32 at a specific location to limit mingling of fluids in the
production stream. By way of example, oil-water interface 32 is
observed either directly or through inference based on computations
as discussed herein.
In an exemplary application, a control algorithm is used to drive
the oil-water interface 32 to a desired interface location. In this
example, it can be assumed that the reservoir in the region of
interest is homogeneous. Also, the array of electrodes 36 is
disposed on the outside of wellbore casing 18, as illustrated in
FIG. 7. Exemplary sensors 36 include one (or two) current providing
(return) electrodes (80). These current electrode(s) are rotated
among sensors 36 so that the remainder of the sensors function as
voltage electrodes. When one current electrode operates as an
injector, the return is at infinity, and the other electrodes
function as voltage measuring electrodes. By definition, the
voltage electrodes draw negligible current. In another mode,
voltages can be maintained at the electrodes, and measured currents
can be injected.
Any substantial change in formation resistivity between the voltage
electrodes 82 is easily detected, because the leakage current from
the wellbore to the formation changes. Because the formation
current is proportional to the gradient in the potential along the
wellbore, any change in the formation current is reflected in terms
of a jump in the derivative of electrical potentials along the
wellbore. The electrodes that straddle this particular region are
sufficient to indicate the region of saturation change and a marker
for this region may be established. In a situation where the
electrodes are kept at a constant potential (and current injected
is measured instead), a jump in the current injected is the
position of the region of saturation change at the wellbore. Thus,
in this situation, it is straightforward to detect the nominal
position of the water encroachment based on the voltage or current
measurements of electrodes at the region of saturation change.
Accordingly, the production rate of oil and/or water can be
adjusted to maintain the oil-water interface 32 at a desired
location.
However, in some environments, a hump or an anomalous rise of
oil-water contact 84 develops in addition to the oil-water
interface surrounding wellbore casing 18, as illustrated in FIG. 7.
The hump may develop away from the wellbore at distances comparable
to the thickness of the formation being produced. Computations have
shown that for a fixed ratio of oil to water production, the
rise-height of the hump 84 is governed predominantly by the oil
production rate. Thus, in such environments, increasing the oil
rate increases the water hump which, upon reaching a certain size,
can lead to breakdown of the dual production system.
It has been determined that the production rates can be controlled
not only for the oil-water contact close to the wellbore but also
for control of hump 84. If a hump is formed by the advancing water,
the rise height of the hump may be an important factor in
observation based control. However, the height of the distant hump
84 can be obtained through data provided by sensors 36. (See FIGS.
8A and 8B).
In this particular example, we can assume that the height of the
water-oil contact close to the wellbore is equal to Z.sub.n and
that the height of the hump 84 is z.sub.f. Based on prior
simulations, the desired position of z.sub.n and z.sub.f, i.e., the
set points, can be labeled as z.sub.sn and z.sub.sf, respectively.
The errors in the near and far rise are established by the
equations
and
If submersible pumps are used for the production of water and oil
as with the electric submersible pumping systems 44 and 48 of FIG.
3, the pumps may be operated to produce a flow rate on the basis of
##EQU1##
where the k.sub.bt term is expected to be small compared to the
first.
The oil rate is controlled by ##EQU2##
where the k.sub.tb term is again expected to be small. All of the k
terms are control constants that will vary depending on the
application and formation but are best obtained by direct flow and
electromagnetics simulation of the reservoir. The k values do not
need to be optimized strictly but rather k values can be selected
that appear to produce a reasonable response. An alternative to the
above equations for controlling the ratio of water to oil rates,
involves directly choosing to control water rates based on the
errors .epsilon.. Also, depending the formation characteristics and
the devices used for producing water and oil (e.g. control valves),
additional or different terms may be required to better approximate
the flow rates required to adequately control the oil-water
interface.
Referring to FIGS. 8A and 8B, examples of actual electrode array
responses are provided that reflect water rise height near the
wellbore (Z.sub.n) and rise height of the hump 84 (Z.sub.f). In
this example, the original oil-water interface was at approximately
1,120 feet and has moved up to 1,115 feet at the wellbore during
production. This change, Z.sub.n is seen as a discontinuity in the
derivative of voltages output by electrodes 82. The computation of
Z.sub.n is straightforward based on the output from sensors 36, as
best illustrated in FIG. 8A.
In this same example, the movement of the hump is detected (and
therefore inverted) from the electrode array data, as illustrated
in FIG. 8B. In this sample, the difference in the computed response
based on output from sensors 36 provides an estimated hump height
change of 5 feet.
However, it should be noted that the discussion above is merely of
exemplary uses of the data provided by sensor array 36. The actual
calculation of a hump height may or may not be necessary, depending
on the particular formation and the production rates. Additionally,
the production equipment, conductivity of the liquids being
produced, formation characteristics, type of sensor array 36, etc.
all affect the formulas, models or direct usage of the output data.
However, the data can readily by adapted to aid in the real time
monitoring and control of fluid production for preventing
intermingling of liquids due to water coning or oil coning.
It will be understood that the foregoing description is of
exemplary embodiments of this invention, and that the invention is
not limited to the specific forms shown. For example, variety of
sensors may be utilized, e.g. a distribution of pressure sensors or
acoustic sensors. Similar to segregated oil/water production, it is
to be understood that a gas/oil interface may be detected (by, for
example, acoustic sensors) and controlled by adjusting gas and oil
rates similar to adjustment of the oil and water rates based on the
equations given above for the oil/water system. Also, the procedure
described above can be further extended to include segregated three
phase production of gas, oil and water. Furthermore, different
types of completions and arrangements of completions can be
utilized to remove oil and water from the formation; and the models
or algorithms used in estimating any changes in liquid production
rates may be adjusted according to the environment and specific
application. These and other modifications may be made in the
design and arrangement of the elements without departing from the
scope of the invention as expressed in the appended claims.
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